The current interest in nonlinear optical (NLO) materials based upon x-electron chromophores stems from the demonstrated possibilities of large nonresonant susceptibilities, ultrafast response times, low dielectric constants, high optical damage thresholds, and the intrinsic tailorability of the constituent structures. When such materials incorporate glassy polymeric architectures, the additional attractive characteristics of supermolecular organization, improved mechanical/dimensional stability, improved optical transparency, and processability into thin-film waveguide structures is possible. Nevertheless, the progression from the above ideas to efficient NLO materials has presented great challenges, and numerous obstacles remain to be surmounted.
For polymer-based second harmonic generation (SHG), X.sup.2 materials, the crucial synthetic problem is to maximize the number density of component high-.beta. chromophore molecules while achieving and preserving maximum acentricity of the microstructure. One early approach to such materials was to "dope" NLO chromophores into glassy polymer matrices and then to align the dipolar chromophore molecules with a strong electric field in a procedure called poling. The performance of such materials is limited by the low chromophore number densities which can be achieved before phase separation occurs and the physical aging/structural relaxation characteristics of all glassy polymers, which lead the randomization of the poling-induced preferential chromophore orientation. Hence, the SHG efficiency of such "guest-host" materials is generally short-lived. In addition, it has been observed that the chromophore constituents are not strongly bound in such matrices and that these materials readily undergo dielectric breakdown during poling. A second approach to the construction of efficient film-based SHG materials has been to incorporate NLO chromophores into Langmuir-Blodgett (LB) films. A priori, such an approach offers far greater net chromophore alignment than is possible in the poling field, where net alignment is statistically determined, temporal stability of the chromophore alignment, and controlled film thickness. While preliminary results with LB film-based NLO have been encouraging, significant problems arise from the fragility of the films, the temporal instability of chromophore alignment, the problem of scattering microdomains, and the structural regularity of layer deposition that is possible.